The phenotypically similar bacteria collectively known as the mutans streptococci are considered major etiologic agents responsible for dental caries. The species most commonly associated with human disease is Streptococcus mutans. Pathogenicity of S. mutans includes the ability to produce antimicrobial substances generally referred to as bacteriocin-like inhibitory substances (BLIS) or bacteriocins. Bacteriocins produced by S. mutans are known as mutacins. These substances are produced by microorganisms to provide a selective force necessary for sustained colonization in a milieu of densely packed competing organisms found in dental plaque.
To date, most bacteriocins remain only partially characterized because they are made in small quantities and only under special cultivation conditions. In addition, mutacins are known to be difficult to isolate from liquid medium. The spectrum of activity and chemical and physical properties of mutacins can vary widely.
Certain bacteriocin peptides or mutacins produced by S. mutans have recently been characterized as belonging to a group of peptides called lantibiotics (Novak, et al. (1996) Anal. Biochem. 236:358-360). Lantibiotics are polycyclic peptides that typically have several thioether bridges, and which can include the amino acids lanthionine or β-methyllanthionine. In addition, lantibiotics can contain α,β-unsaturated amino acids such as 2,3-didehydroalanine and 2,3-didehydro-2-aminobutyric acid, which are the products of post-translational modification of serine and threonine residues, respectively.
Certain lantibiotics have demonstrated antibiotic activity, mainly against Gram-positive bacteria (Bierbaum and Sahl (1993) Int. J. Med. Microbiol. Virol. Parasitol. Infect. Dis. 278:1-22). Nisin and epidermin are the best known examples of the 20 or so lantibiotics which have been identified to date. They are ribosomally synthesized as prepropeptides that undergo several post-translational modification events, including dehydration of specific hydroxyl amino acids and formation of thioether amino acids via addition of neighboring cysteines to didehydro-amino acids. Further post-translational processing involves cleavage of a leader sequence, which can be coincident with transport of the mature molecule to the extracellular space. A mature lantibiotic molecule is usually about 20 to 35 residues having thioether linkages that result in cyclical segments and provide a substantial degree of rigidity to the rodlike structure.
Current evidence indicates that the biological activity of certain lantibiotics, e.g., those known as “type A” lantibiotics, depends on the association of a number of molecules with the membrane of a target bacterium to form ion channels, thereby resulting in desynergization. Rapid loss of all biosynthetic processes occurs, resulting in death of the target cell. Other lantibiotics known as “type B” lantibiotics, can exert their effect by specifically inhibiting certain enzymes.
The genetics of lantibiotic production have been studied in several species of bacteria. In general, it has been found that a structural gene for a preprolantibiotic is clustered with genes that encode products responsible for post-translational modifications of the lantibiotic. In certain instances, these genes are known to form an operon or operon-like structure (see e.g., Schnell, et al. (1992) Eur. J. Biochem. 204:57-68). Production of lantibiotics can also require accessory proteins, including processing proteases, translocators of the ATP-binding cassette transporter family, regulatory proteins, and dedicated producer self-protection mechanisms. For example, at least seven genes have been shown to be involved in epidermin biosynthesis.
Lantibiotic properties have been exploited in certain products that are commercially available. The lantibiotic nisin has been developed as a food preservative that has been given “Generally Recognized as Safe (GRAS)” status by the federal Food and Drug Administration (FDA). It is employed as a food preservative in more than 40 countries and is used in preference to nitrites and nitrates. The oral toxicity of this compound, and presumably other lantibiotics, is very low in rats (LD50=7 g/kg; Hurst, (1981) Adv. Appl. Microbiol. 27:85-123). Other applications for nisin, including its use as a mouth rinse (Howell et al, (1993) J. Clin. Periodontal 20:335-339), are actively being examined by a large number of laboratories.
The discovery of new lantibiotic compounds having antibiotic activity can be particularly important in view of the increased resistance to presently available antibiotics in certain pathogenic microorganisms. Novel lantibiotic compounds having unique or superior activity against particularly virulent pathogenic bacteria are desirable in providing new weapons in the arsenal against bacterial infection.